(a) Field of the Invention
The present invention relates to a control device for controlling a switching operation, an LED light emitting device including the same, and a control method.
(b) Description of the Related Art
A buck converter includes a bridge diode for rectifying an AC input. Particularly, the buck converter having no bulky capacitor varies a rectifying diode output voltage according to a load connected to the buck converter. The rectifying diode output voltage is generated by rectifying AC power that is input to the buck converter. The rectifying diode output voltage will be referred to as an input voltage hereinafter.
Also, the load connected to the buck converter depends on a switching operation of a power switch for controlling the buck converter. When the power switch (S) is not switched, input impedance of the buck converter is very large, and hence an input voltage has a constant DC offset voltage. The DC offset voltage applies a negative influence to detection of an input voltage or phase.
FIG. 1 shows an LED light emitting device for emitting a plurality of LEDs by supplying current to an LED string including a plurality of LEDs using a buck converter. In FIG. 1, the rectifying diode is realized with a bridge diode 14.
As shown in FIG. 1, in the LED light emitting device, AC power (AC) is rectified by passing through the bridge diode 14. The bridge diode 14 full-wave rectifies the input AC power. The rectified voltage, that is, the input voltage, is supplied to the inductor 11, and the inductor 11 supplies a driving current to a plurality of LEDs according to an operation of the power switch (S). The switch 15 including the power switch (S) controls a switching operation of the power switch (S).
When the power switch (S) is turned on, inductor current (IL) flowing to the inductor 11 is increased, and when the power switch (S) is turned off, the inductor current (IL) is reduced. The peak value of the inductor current (IL) for a single switching period follows the full wave rectified current having passed through the bridge diode 14. Therefore, the peak value of the inductor current (IL) follows the full wave rectified sine wave, and the inductor current (IL) rises up to the peak value and then falls periodically. The duty of the power switch (S) is determined by the input voltage. In detail, the duty is reduced as the input voltage is increased and the duty is increased as the input voltage is reduced in order to maintain output power.
Information on the input voltage is needed to control the switching operation of the power switch (S). A voltage at an input end of the power switch (S) electrically connected to the bridge diode 14 has a waveform similar to the input voltage.
In this instance, if the switch 15 controls the switching operation of the power switch (S) while the input voltage does not become a zero voltage because of load variation or noise in the input AC power, the switch 15 controls the power switch (S) based on the wrong input voltage information by a DC offset voltage of the input voltage. When the power switch (S) is mistakenly controlled, noise occurs by the switching operation of the power switch (S), and the input voltage is not reduced to be near 0 volts.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.